Globe anchors don't update when using timesampled attributes

[lilleyse]

Globe anchors don't update when using timesampled attribute since these don't trigger USD change notifications.

To fix this, we need some other way of detecting when a timesampled attribute has changed.

[ckolve]

Potentially this could be done via dedicated action graph nodes (the input would be lat/long/elevation via a readPrimAttrib and the output xyz, to be written via a writePrimAttrib?) there is precedence for this when looking at how animation curves are being handled. I also noticed crashes when repeatedly setting anchor lat/long values in fast succession (via a script or omnigraph), which might be related to how changes are triggering updates.